1. Field of the Invention
The present invention generally relates to a method for determining the concentration of polycarboxylic acids in a solution. More particularly, an embodiment of the invention relates to a receptor for polycarboxylic acids and a method for using the receptor for determining the concentration of a polycarboxylic acid in a solution.
2. Description of the Related Art
Selective binding of ions in highly competitive media, such as water, has intrigued chemists for many years (Cram and Trueblood, Top. Curr. Chem., 98:43, 1981). The binding of biologically active anions, e.g., phosphates or carboxylates, has been one focus of interest for investigations which aim to mimic enzymes or transport proteins (Kneeland et al., J. Am. Chem. Soc., 115:10042, 1993; De Mendoza et al., Top. Curr. Cyhem., 175:101, 1995). These investigations are typically focused upon binding strength and selectivity. The first may be regulated by the choice of functional groups or solvent systems. The second may be achieved by introducing elements complementary to the shape and binding characteristics of the substrate (Lehn, Supramolecular Chemistry, Concepts and Perspective, VCH, New York, 1995; Cram, Angew. Chem. Int. Ed. Engl., 27:1009, 1988).
The field of molecular recognition is becoming sophisticated enough that both binding strength and selectivity may be tuned for many classes of guests (Schneider, Angew. Chem. Int. Ed. Engl., 30:1417, 1991). Strong and selective binding of hydrophilic guests, however, in high dielectric media is still elusive.
Citrate, a tricarboxylic acid at near neutral pH, is hydrophilic and highly charged. Citrate is also of commercial interest because of its abundance in citrus fruits and common beverages. Citric acid containing fruit is widely produced. Further, citrate is often listed as one of the top five ingredients in common lemon/lime flavored beverages. Citrate is also a relatively simple molecule, having three carboxylic acids emanating from a central carbon. Citrate's charge is minus three (near neutral pH) and therefore is distinctive compared to other possible interfering species present in beverages such as simple salts and sugars. Hence, if a receptor complementary to both the charge and hydrogen bonding ability of citrate were to be developed, potential interference from competing analytes would not be a concern in a food industry application.
Design principles such as preorganization, hydrogen bonding, and charge pairing may be tested within the context of achieving selective and strong binding of polycarboxylic anions in water. These are well accepted molecular recognition paradigms, but they have been utilized under different conditions than those used herein. For example, although the benefits of preorganization are documented (Lehn, Supramolecular Chemistry, Concepts and Perspective, VCH, New York, 1995; Cram, Angew. Chem. Int. Ed. Engl., 27:1009, 1988), they have been mostly conducted in low dielectric media or with studies of hydrophobic interactions in water. Contradictory results are found for hydrogen bonding groups for phosphates and carboxylates in water. For instance, ammonium groups are more effective for charge pairing than guanidiniums due to the higher localization of charge (Dietrich et al., Helv. Chem. Acta., 62:2763, 1979), yet nature exhibits a preference to use arginine rather than lysine to bind these anions, possibly due to an increase in the number of possible hydrogen bonds (Hannon et al., Bioorganic Frontiers, Springer Verlag, Berlin, 3:143-256, 1993). It is also generally found that the higher the charge on the host or guest the larger the binding constant (Dietrich, et al. Helv. Chem. Acta., 62:2763, 1979). The field of molecular recognition is sophisticated enough that applications for rationally designed and totally synthetic receptors are realistic (Morttellaro and Nocera, Chemtech, 26:17, 1996; Diamond and McKervey, Chem. Soc. Rev., 16, 1996). Although the field has primarily focused upon understanding non-covalent interactions such as hydrogen bonding and hydrophobic effects (Peterson et al., Tetrahedron, 51:401, 1995; Perreault et al., Tetrahedron, 51:353, 1995), recent effort has been put toward the development of sensors for various analytes. As a few examples, sensors for sugars (James et al., Angew. Chem. Int. Ed. Engl., 33:2207, 1994), Zn(II) (Godwin and Berg, J. Am. Chem. Soc., 118:6514, 1996), creatine (Bell et al., Science, 269:671-674, 1995), BTXs (Pikramenou et al., Tetrahedron Lett., 34:3531, 1993) and cAMP (Adams et al., Nature, 349:694, 1991) have been developed. The promise of this field is highlighted by the recent establishment of a World Wide Web site soliciting sensors (http://www.curscl.co.uk/BioMedNet/cmb/cmbinf.html).
Analytes are sensed when they physically bind to a properly designed receptor and produce a measurable signal upon complexation (Fabbrizzi and Poggi, Chem. Soc. Rev., 200, 1995). Therefore a successful sensor system will possess both a "host," developed via molecular recognition principles, and a vehicle for producing a signal. Spectroscopic or electrochemical changes as a function of the analyte have been the most popular vehicles for qualitative and quantitative assays. Although absorption spectroscopy and electrochemical changes are useful for certain applications, fluorescence spectroscopy offers advantages over these techniques (Czarnik, Chemistry and Biology, 2:423, 1995). Fluorescence emission appears at longer wavelengths than fluorescence excitation and the background signal is typically low, resulting in high sensitivity.
Using antibodies in a sensing scheme is well developed method for immunoassay technologies (Birch and Lennox, Monoclonal Antibodies: Priciples and Applications. John Wiley & Sons, New York, 1995). These assays typically rely on a competition approach. Addition of a solution to be analyzed containing an unlabeled antigen results in the release of a labeled antigen and hence a signal (FIG. 1, Birch and Lennox, Monoclonal Antibodies: Priciples and Applications. John Wiley & Sons, New York, 1995). This approach is particularly amenable to synthetic receptors, possibly resulting in sensors for a wide variety of analytes.
Polycarboxylic acids are frequently found in food products. Polycarboxylic acids include a wide variety of compounds containing at least two carboxylic acid groups. A number of polycarboxylic acids are present in food products including, but not limited to, maleic acid, ascorbic acid, and citric acid. In general it is desirable to design a variety of receptors for a variety of polycarboxylic acids. Such receptors would be useful for determining the concentration of some of polycarboxylic acids in food products.